People around the world now use Voice-Over-Internet-Packet (VoIP) services and other IP services, such as text messaging and video conferencing, as means of communication. In some cases, this is their primary or only means of communication from some locations. People are also accustomed to being able to contact local emergency services by, for example, dialing 911. Issues presented by the combination of IP services and emergency services include, for example, determining the location of callers and routing emergency phone calls to appropriate emergency call-takers.
Systems and standards are being developed for the combination of emergency services and VoIP networks. In the United States, for example, the National Emergency Number Association (NENA) has taken initiative to lay out the future steps for the evolution of Enhanced 9-1-1 (E9-1-1). Another example, Next Generation E9-1-1 (NG E9-1-1), the name for the long term solution that is completely IP based, supports requirements of traditional systems as well as new features made feasible by transitioning to IP.
One protocol used, for example, to set up and tear-down network communications is Session Initiation Protocol (SIP). SIP can be used for creating, modifying, and terminating sessions with one or more participants and for creating two-party, multiparty, or multicast sessions that include Internet telephone calls, multimedia distribution, and multimedia conferences. SIP is independent of the underlying transport layer and it can run on, for example, Transmission Control Protocol (TCP), User Datagram Protocol (UDP), or Stream Control Transmission Protocol (SCTP). One element of a SIP system is a SIP proxy server that can be used to, for example, route requests to a user's current location, authenticate and authorize users for services, implement provider call-routing policies, and provide various features to users. SIP also defines various messages types that can currently be found in RFC 3261, available from the Internet Engineering Task Force (IETF).